I think (although I may be wrong as I have no real technical knowledge) that you have 3 variables to think about- speed, weight and altitude. Ideally, you would want to keep the speed and altitude at the optimum numbers, as you can't control the weight. That would give a cruise climb, as you burn off fuel, you're optimum level increases, you slowly climb (assuming that there is a single optimum cruise mach), but because you can't climb continuously, you have to stay at a level and burn off fuel. The FMC makes the best of this by steadily reducing mach number. Once you step-climb to a higher level, the mach increases toward the "best" speed (perhaps faster if above the "optimum" level) then reduces as fuel burns off. At least, that's my understanding of it.